Make virtio vq size configurable by a guest.

[Gleb Natapov <gleb@redhat.com>]

Hi,

Currently in virtio host dictates the size and layout of vq that should
be used.  To talk to a device that has one vq with 128 elements guest
needs to allocate at least 2 pages. Usually this is not a problem, but
sometimes guest runs in a resource restricted environment and then it may
not have enough memory to initialize all virtio devices present in the
system. One such environment is a BIOS. Seabios currently has virtio block
support. Since the BIOS should be able to access the disk even after OS
is launched vq should be allocated from a special memory region that will
be marked as unavailable to an OS, but such memory is scarce. Because vq
is so huge only a couple of virtio disks can be initialized by the BIOS.

It would be nice if a guest will be able to tell to a host what vq size
should be used instead. BIOS issues only one request at a time anyway,
so it needs only one element in vq. It does not care about performance
to much either, so it can tell to a host to not align used index to a
page boundary. This way vq of one element shouldn't take more then a couple
hundreds of bytes.

--
			Gleb.